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Identifying problem with a cheap SMPS.

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4kruby:
Hello,

I had a cheap SMPS wall adapter (9v 1A).
When it is powered, the output is around 10.05 V. But I am getting earthing (shock) on the barrel connector of the power supply. AC tester glows pretty brightly when touched on the barrel connector.
I pried open the power supply and found one of the worst PCBs inside it (attached images).

Can anyone point out where the problem is (as to why am I getting the leakage)?

Note : I can see that R8 is damaged.
Edit : R8 has no connectivity (and shows as open circuit). Is this the problem?

Thank you.

rob77:
R8 is a fusible resistor, and seems like someone by-passed it after it was blown (the red wire soldered after the R8) - mains input should go to the 2 points on the PCB. also seems the capacitor across the primary and secondary is not a proper Y class .. i would throw away the whole thing.

4kruby:
Thanks. I am not going to use it anyway as it gave me a shock when I first used it months ago.

It's a new SMPS and i opened it for the first time. I think the manufacturers itself did this.

which is the Y capacitor are you referring to? is it C5?

Gyro:
The Y cap (if it is actually a safety rated) is the blue one alongside the transformer, I can't see the part number or device marking in any of the photos. C5 looks to be part of a snubber on the primary side.

It's likely that the shock you feel is Y cap (?) leakage, but the quality of construction is so poor that you cannot rely on the rest of the construction to be safe (eg. the transformer).

Specific observations:

1. Bypassed blown fuse resistor on mains input (who would do that?  :o)
2. Incorrect PCB clearances between primary and secondary side.
3. Incorrect PCB clearances primary side (especially now the fuse resistor is bypassed).
4. Board wrapped in flammable expanded foam, the PCB obviously wasn't designed for that case.
5. Very poor soldering - solder splashes where there really shouldn't be solder splashes.
6.  Bodged components, eg, two diodes in parallel for the output rectifier - they will not share current properly, the hotter one will pass more current.

Is there any way you can report the manufacturer to the authorities? At best, you should destroy it and crush the PCB to ensure that it is not salvaged and re-used (again?).

HumbleDeer:
I agree with Gyro above;

SMPS without an earthing terminal usually have output ground tied to mains neutral with a Y cap, for interference suppression reasons, compensating for an "invisible" capacitor inside the transformer. The cap provides a return path essentially, so it doesn't have to try and find it through your equipment.

Some cheap supplies don't use a Y cap, and instead just use a high voltage capacitor. This is incredibly dangerous.

If it is a proper Y cap, not damaged, and the SMPS is otherwise not damaged, then the remaining reason could be a high capacitance. Higher capacitance has more potential for suppression, but also leaks more AC current into the Output negative.

One note against Gyro's comment:

--- Quote from: Gyro on January 21, 2022, 05:47:21 pm ---5. Very poor soldering - solder splashes where there really shouldn't be solder splashes.

--- End quote ---
Solder splashes where there really shouldn't be solder splashes? So, everywhere? There are no places where solder splashes, potentially shorting things, are appropriate.  ;D

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